Commercial siloxane polymers are generally produced by the hydrolysis of dichlorosilanes, yielding a mixture of cyclic and linear polysiloxanes. The production of cyclic polysiloxanes can be enhanced relative to linear polysiloxanes by performing the hydrolysis in a highly dilute solution, which improves the probability of cyclization versus oligomerization.
The preparation of poly(organohydrosiloxanes) is of particular interest due to the reactivity of the silicon-hydrogen bond. Organosilicon compounds can be produced by reaction of olefins with the .ident.SiH groups. The production of cyclic poly(organohydrosiloxanes), however, is especially difficult via the hydrolysis reaction due to the high reactivity of the silicon-hydrogen bond.
Several methods other than hydrolysis have been reported to produce cyclic polysiloxanes. Most prior processes involve the base-catalyzed thermal cracking of poly(diorganosiloxanes). Cyclic polysiloxanes have been prepared by Hunter et al., Journal of the American Chemical Society, 68, 667, (1946) via the cracking of linear polysiloxanes in the presence of a base catalyst, such as sodium hydroxide. Okamato et al., U.S. Pat. No. 3,989,733, describe a process for the thermal cracking of poly(diorganosiloxanes) in the presence of lithium hydroxide to produce cyclic poly(diorganosiloxanes). A process for the catalyzed cracking of silicone polymers is disclosed by Greenlee, U.S. Pat. No. 5,110,972. High molecular weight silicone polymers are dissolved in an organic solvent containing sulfuric acid at 150.degree.-180.degree. C., and cracked to lower molecular weight linear species. This is followed by the addition of potassium hydroxide and the distillation of the cyclic products from the solution. Kuznetsova et al., U.S. Pat. No. 3,558,681, describe the preparation of methylphenylcyclotri- and tetrasiloxanes by the vapor phase rearrangement of the linear siloxane at 250.degree.-360.degree. C. using lithium hydroxide or lithium silanolate as the catalyst. While cyclic poly(diorganosiloxanes) have been prepared with varying degrees of success via base-catalyzed routes, the preparation of cyclic poly(organohydrosiloxanes) via these methods would be unacceptable. The base-catalyzed reaction is constrained by the potentially violent reaction of poly(organohydrosiloxanes) with alkali metal compounds and the formation of higher molecular weight gels through silicon-oxygen-silicon bond formation.
Alternatively, cyclic siloxanes have been prepared by the acid-catalyzed depolymerization of linear siloxanes. Burkhardt et al., U.S. Pat. No. 4,276,425, describe a process for preparing cyclic poly(dimethylsiloxanes) through the reaction of linear poly(dimethylsiloxanes) with 50-85% aqueous sulfuric acid at 130.degree.-150.degree. C. for 1.5 to 6 hours. The cyclic species and water are recovered by distillation. While an 89% yield of cyclic species is claimed (.about.3/1 octamethylcyclotetrasiloxane/decamethylcyclopentasiloxane), this process would not be feasible for the large scale production of cyclic organohydrosiloxanes due to the inherent explosive risk of the contact of large amounts of hydrosiloxanes with highly concentrated sulfuric acid. Another drawback is the corrosive nature of the reaction mixture and additional equipment wear associated with the handling of concentrated sulfuric acid solutions.
Cyclic polysiloxanes, including silicon-hydrogen bond-containing cyclics, were prepared by Miller et al., U.S. Pat. No. 3,714,213, using sulfuric acid-treated clay to crack linear poly(methylhydrosiloxanes) in a pot distillation column at .about.300.degree. C. While cyclic products were formed, this process would not be commercially viable due to the prolonged exposure of the reactant linear hydrosiloxanes to heat and acidity. The large amounts of evolved hydrogen gas and the loss of yield due to the formation of high molecular weight material would make the scale-up of such a process difficult and potentially dangerous.
Crivello et al., U.S. Pat. No. 4,895,967, disclose the incremental addition of linear polysiloxane, including poly(methylhydrosiloxane), to a hot (200.degree.-800.degree. C.) acid catalyst under reduced pressure. The resultant volatile cyclic compounds are recovered through condensation. Although the semi-batch nature of this process reduces the potential risk of a build-up of linear hydrosiloxanes in contact with heat and acidity, the risk still remains. High molecular weight nonvolatile species will tend to stay on the catalyst surface and gel through silicon hydride dehydrogenation over the course of the reaction, thus deactivating the catalyst surface. This would produce hydrogen and allow the build-up of linear hydrosiloxanes. An additional disadvantage is that the incremental addition of the reactants is an inefficient and costly utilization of reactor volume and catalyst active surface area, since the reactants are not exposed to the entire catalyst bed. Since the residence time on the catalyst is fixed, the flexibility of this process to produce various amounts of different cyclic species is limited.
Processes disclosed up to now for the production of cyclic organohydrosiloxanes have employed process conditions and materials that are costly, potentially hazardous, and offer little flexibility in producing various cyclic species. Therefore, it would be advantageous to provide a process that could produce cyclic hydrosiloxanes and/or cyclic diorganosiloxanes safely and in high yield, with the flexibility to vary the product distribution.